The invention relates to a process for concentrating solid, liquid and gaseous waste products which arise during the manufacture of carbon electrodes, and relates also to an adsorbent for dry scrubbing equipment for cleaning fumes released from equipment for manufacturing carbon electrodes.
Carbon electrodes manufactured on an industrial scale, in particular anodes for producing aluminum by the fused salt electrolytic process, are made from petroleum coke, anode butts and pitch. After drying, grinding, sieving and mixing, these materials are shaped into blocks of, for example, 400-1200 kg, then baked in a firing process at about 1100.degree. C. to give the final electrode.
At various stages of manufacture of carbon electrodes waste products in solid, liquid and/or gaseous form are produced. Taking the example of anodes for the aluminum industry the following examples can be given:
(a) Waste from the cleaning of the anode butts: coarse and fine particulate carbon, mixed with alumina, cryolite, iron and silicon.
(b) Dusts created during crushing operations: fine particulate petroleum coke with varying fractions of alumina, silicon, iron and other impurities, to some extent including considerable amounts of sulphur.
(c) Tar and oily substances.
(d) Gaseous emissions: non-condensed hydrocarbons, fluorides and sulphur dioxide.
Up to now these waste products have usually been recirculated, burnt off and/or released to the air.
For recirculation purposes use is made in particular of electrostatic filters which effect the precipitation of tar and oily substances out of the waste gases emerging from the electrode baking furnaces. Dust is also removed from the air stream by means of suitable mesh filters and returned to the process. The materials extracted in the recirculation process can however have a negative effect on the quality and constancy of the product. Disposal by combustion of waste products by third parties, dumping or direct emission can lead to high costs as ever stricter regulations have to be satisfied.
A dry scrubbing unit using aluminum oxide as adsorbent has been proposed for the removal of fluorine, fluoride and tarry residuals. Using such an adsorbent it is possible, as is known from aluminum smelter operations, to remove fluorine and tarry components; it is not possible, however, to remove sulphur and sulphur-containing components.
These are two particular disadvantages associated with the use of aluminum oxide as an adsorbent for scrubbing the fumes released from plants manufacturing carbon electrodes:
Not all of the waste products are absorbed that are released during anode production and pollute the atmosphere.
Because of the large amounts of aluminum oxide required, an aluminum smelter must be situated close by. Even if this is the case, the alumina which is contaminated with tar residuals presents a significant disadvantage in subsequent operations.